Pressure monitoring system and inflation valve using same

CROSS REFERENCE TO RELATED APPARITIONS

This is a Continuation of International Application PCT/GB00/02929, with an International filing date of Jul. 28, 2000, that designates the United States and claims priority from GB application 9917579.6 filed Jul. 28, 1999.

FIELD OF THE INVENTION

This invention relates to a pressure monitoring system and, in particular, a system for monitoring the air pressure within a pneumatic wheel.

DESCRIPTION OF RELATED ART

A variety of pressure monitors have been proposed. U.S. Pat. No. 5,289,160 discloses a tire pressure monitoring system in which a pressure sensor switch closes if the tire pressure falls below a given valve relative to the external ambient pressure to trigger a warning system. The pressure sensitive switch comprises biasing means the action of which is resisted by the tire pressure but if the tire pressure falls too low relative to atmospheric pressure, the biasing means causes an electrical switch to dose so as to generate a warning signal. This system uses a surface acoustical wave (SAW) device as a transponder to transmit the warning signal to a remote sensor. The possibility of using a SAW device directly on a diaphragm to measure deflection of the diaphragm and thus provide a measure of the pressure acting on the diaphragm, relative to atmospheric pressure, is also mentioned.

U.S. Pat. No. 5,585,571 discloses apparatus for measuring strain by means of a pair of SAW devices. In one embodiment SAW devices are mounted on opposite sides of a diaphragm which is exposed to atmosphere on one side and to tire pressure on the other side. The signals from the two SAW devices are then mixed to provide a temperature compensated signal indicative of the tire pressure.

The present invention aims to improve upon these known systems.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provided a pressure monitoring system comprising a first pressure chamber and a second pressure chamber separated by a diaphragm, a first SAW device having a first resonant frequency mounted on one side of the diaphragm and a second SAW device having a second resonant frequency mounted on the other side of the diaphragm, the first chamber being arranged to be pressurized to a predetermined pressure P₁and to remain at that pressure and the second chamber being arranged to initially be pressurized to a pressure P₂equal to pressure P₁but being subject to change over time, whereby if the pressure P₂in the second chamber falls or rises relative to the pressure P₁in the first chamber, the diaphragm will deflect so one side thereof is subject to tension and the other side thereof is subject to compression, the first and second SAW devices being arranged to sense this deflection by sensing said tension and compression and hence providing an output indicative of the change of the pressure P₂in the second chamber relative to the pressure P₁in the first chamber.

Thus, the SAW devices can be used to provide a measure of the pressure differential between the two chambers, and the pressure in the first chamber can be set to, in effect, store the desired pressure. Thus, if the pressure in the second chamber (which may, for assistance, be in communication with the air pressure within a pneumatic wheel), falls relative to this stored pressure, a warning signal is generated by the pair of SAW devices. Furthermore, the SAW devices provide a temperature compensated signal and can be used as a transponder to transmit the warning signal to a remote sensor.

Preferred and optional features of the invention will be apparent from the following description and from the subsidiary claims of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, merely by way of example, with reference to the accompanying drawings, in which:

FIGS. 1A, <b>1</b>B and <b>1</b>C show passive strain sensitive SAW devices which may be used as the diaphragm in a system according to the invention;

FIG. 2 is a cross-section of a first embodiment of a pressure monitoring system according to the present invention;

FIG. 3 is a schematic diagram illustrating how such a system may be used in practice; and

FIG. 4 is a cross-sectional view of a valve incorporating such a pressure monitoring system.

FIG. 5 is a schematic diagram showing a sensor/transponder of the pressure monitoring system for use on vehicles in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1A, <b>1</b>B and <b>1</b>C show first and second SAW devices <b>1</b>, <b>2</b> mounted on opposite sides of a central substrate which may be used as a diaphragm. Each SAW device is housed within a cover <b>4</b>, <b>5</b> which protects it from the environment and is mounted on opposite sides of a piezoelectric substrate <b>6</b> (such as quartz or thin film zinc oxide) as shown in FIG. <b>1</b>A. The properties of the central substrate, e.g. it stiffness and/or electrical conductivity, can be altered by using a multiple layer substrate. FIG. 1B, shows the use of single-sided SAW devices mounted on piezoelectric substrates <b>6</b>A, <b>6</b>B mounted back to back and FIG. 1C, shows an embodiment in which a further structural layer <b>6</b>C e.g. a metal or plastics layer, is used between the piezoelectric substrates <b>6</b>A and <b>6</b>B.

FIGS. 1A, <b>1</b>B and <b>1</b>C also show electrical connection pads <b>3</b>A and <b>3</b>B for connecting the SAW devices to an aerial and to a ground plane.

These devices may be fabricated and packaged in manner as described in our copending application No. GB9902341. 8.

The use of two SAW devices, one on each side of the diaphragm, enables them to be operated as a differential strain sensor in the manner described in U.S. Pat. No. 5,585,571 the teaching of which is incorporated herein. As described therein, the two SAW devices are designed so they have slightly different resonant frequencies. By subtracting the output of one SAW device from the other a measurement of the differential strain can be determined and the output is temperature compensated as changes with temperature are effectively cancelled out. No separate measurement of temperature is thus required.

This arrangement contrasts with many conventional arrangements of SAW devices in which one device is used as a measuring element and another is used as a reference. With the differential arrangement, there is no need for a reference SAW and the sensitivity is increased as the two devices are subject to equal and opposite strains. Also, the diaphragm is normally unloaded (when the pressures on opposite sides thereof are equal) so avoiding ageing problems and enabling the measurement of higher pressures.

FIG. 2 shows an embodiment of the invention in which a diaphragm <b>7</b> is provided with a SAW device <b>8</b>, <b>9</b> on opposite sides thereof, for example in the manner described in relation to FIGS. 1A, <b>1</b>B and <b>1</b>C, and mounted within a housing <b>10</b>, e.g. formed of plastics. The housing <b>10</b> defines a first chamber <b>11</b> on one side of the diaphragm <b>7</b> and a second chamber <b>12</b> on the other side of the diaphragm. The diaphragm separates the two chambers <b>11</b> and <b>12</b> and provides a seal therebetween. The first chamber <b>11</b> is closed by a one-way valve <b>13</b> which lets air (or other gas) into the chamber <b>11</b> but prevents air from escaping from the chamber <b>11</b>. The second chamber <b>12</b> is in communication with the pressure to be monitored.

The one-way valve may be a simple disk with a slot in it arranged to open when pressure is applied to one side thereof but other types of one-way valve may be used.

FIG. 3 is schematic diagram illustrating the manner in which the pressure sensing system operates. It shows the diaphragm <b>7</b>, housing <b>10</b> and chambers <b>11</b> and <b>12</b> as described above. The chambers <b>11</b> and <b>12</b> may be pressurized by a source of pressured air via conduits <b>14</b> and <b>15</b>, with a non-return valve <b>13</b> shown in conduit <b>14</b>.

When the system is first pressurized, the pressure P₁in chamber <b>11</b> and the pressure P₂in chamber <b>12</b> will be equal. Chamber <b>11</b> then, in effect, stores the value of P₁as the one-way valve <b>13</b> prevents air escaping from chamber <b>11</b>. Chamber <b>12</b>, on the other hand, is in communication with the pressure to be monitored, e.g. the tire pressure, which may be subject to variation, e.g. due to slow leaks or puncture. If the pressure P₂in chamber <b>12</b> falls relative to pressure P₁in chamber <b>11</b>, the diaphragm <b>7</b> will deflect and the SAW devices will sense this and provide a warning signal.

Such an arrangement can be used to monitor tire pressure. When the tire is initially inflated, chamber <b>11</b> stores the initial pressure and if, over time, the pressure P₂in chamber <b>11</b> falls relative to this stored pressure a warning signal is generated. If the tire is over-inflated initially, this simple embodiment would store the incorrect, over inflated pressure in chamber <b>11</b>. However, this problem can be avoided by using an inflator which can be pre-set to inflate the tire to a selected pressure.

FIG. 4 is a cross-sectional view of an embodiment of a valve, such as a vehicle wheel valve, incorporating a pressure monitor as described above. The pressure monitor is mounted at the inner end of the valve body <b>18</b>. A first Schraeder valve <b>19</b> is provided to function as the one-way valve <b>13</b> shown in FIG. 3 enabling air to enter the first chamber <b>11</b> but not to escape therefrom. A second Schraeder valve <b>20</b> is provided as in a conventional inflation valve for a pneumatic tire to allow the tire to be inflated but prevent air escaping through the valve except by activating a release rod <b>21</b> by means of which the valve can be opened for two-way communication. It is advantageous to link the two Schraeder valves <b>19</b> and <b>20</b> so that actuation of the release rod <b>21</b> also actuates a release rod <b>22</b> of the first valve <b>19</b> via connecting rod <b>23</b>. Thus, if the tire is over inflated and air is let out through valve <b>20</b> by actuating release rod <b>21</b>, the valve <b>19</b> will also be opened to allow air to escape from chamber <b>11</b> so that it does not store the over pressure.

It will be appreciated that the pressure monitor device can be easily re-calibrated by simply inflating the tire to the required pressure as the first chamber <b>11</b> automatically stores the peak inflation pressure.

The arrows shown in FIG. 4 show the path of air used to inflate the tire and pressure chamber <b>11</b> at the same time.

An advantage of using SAW devices on piezoelectric substrate is that they can store and convert energy without the requirement of an external power source such as a battery and wireless radio frequency signals can be used to interrogate the devices as described in U.S. Pat. No. 5,585,571. Also, as shown in FIG. 5, the SAW devices or transponders <b>112</b> need only be connected to an aerial and a ground plane to enable them to transmit signals to a remote sensor <b>34</b>, no other circuitry being required. If desired, a small processing circuit (not shown) may be mounted within the valve body <b>18</b> to process the signals received from the SAW devices. FIG. 4 shows metal insert <b>24</b> running the length of the valve body <b>18</b>, which provides an aerial. A ground plane connection is provided by a metalized ring <b>25</b> around the body <b>18</b> which, in use, contacts a metal wheel <b>36</b> (FIG. 5) in which the valve <b>18</b> is inserted, (the electrical connections between these components and the connection pads <b>3</b>A, <b>3</b>B of the SAW devices are not shown in this figure).

Signals from the SAW devices can thus be transmitted to remote sensors <b>34</b>, e.g. sensors built into the vehicle in the vicinity of the wheels and arranged to provide a warning signal on the vehicle dashboard. The SAW devices may also be arranged so that each sensor provides an unique output identifying which sensor is producing a warning signal so that the warning can indicate which wheel or wheels is causing a problem. The monitor described above can thus provide an inexpensive, compact device for measuring tire pressure for a tire <b>38</b>. The device is so small and lightweight that its effect on the performance of the wheel is negligible. Yet it can provide warning of low pressure in a tire and is capable of sensing a small drop in pressure, e.g. of 1 or 2 pounds per square inch (6.9-13.8×10³NT/M²), so can provide a warning of problems such as a slow puncture. It can also provide a warning of a sudden loss of pressure, e.g. due to a puncture, in situations where it is not immediately apparent to a driver, e.g. of a lorry tire, trailer tire or of a spare tire.